High dynamic signal Doppler frequency shift estimation and error estimation method based on ephemeris

Abstract

The invention discloses a high-dynamic signal Doppler frequency shift estimation and error estimation method based on ephemeris, and belongs to the technical field of satellite communication. The method comprises the following four processing steps: determining an ephemeris model and input parameters thereof, predicting the position and speed of a communication window period according to the ephemeris model, estimating Doppler frequency shift, and analyzing Doppler frequency shift estimation errors. The method is suitable for a low-orbit satellite communication system, Doppler frequency shift estimation and error analysis of low-orbit satellite-ground communication and inter-satellite communication are supported, the error analysis process is easy to implement, and error analysis can be independently carried out without depending on ephemeris model prediction in a typical scene.

Description

Technical Field

[0001] The invention relates to a high dynamic signal Doppler frequency shift estimation and error estimation method based on ephemeris, belonging to the technical field of satellite communication. Background Art

[0002] In low-orbit satellite communication systems, since the satellite platform flies around the earth at high speed under the influence of the earth's gravity, the signal transmitter and receiver are not in a relatively static state. A high dynamic environment refers to a violent relative motion between the transmitter and the receiver, which can cause a significant Doppler shift. When the receiver moves toward the transmitter, the received signal frequency will increase; otherwise, it will weaken. The Doppler shift caused by the high dynamic environment belongs to a fast time-varying fading channel. At this time, the constellation diagram of the signal will rotate during the transmission process, and the angular velocity of the rotation is not a constant value. When the rotation angle exceeds a certain threshold, the symbol mapping polarity will be reversed, increasing the bit error rate of the data decoded by the received signal, and even demodulation lock loss and interruption of communication. In order to ensure normal communication and improve communication quality, measures must be taken to compensate and offset the Doppler shift.

[0003] Fast and accurate estimation and compensation of Doppler shift is one of the key technologies in low-orbit satellite communication systems. The magnitude of Doppler shift is related to the carrier frequency, relative speed, and the angle between the communication link and the relative motion direction. Among them, the carrier frequency is a known communication waveform parameter, and the remaining two parameters can be obtained through ephemeris. Ephemeris is a series of data tables or databases about satellites that provide information such as the position and displacement of satellites in their orbits. Using ephemeris to estimate Doppler shift is an effective technical means.

[0004] There are inevitably errors in ephemeris prediction, and there are also errors in Doppler frequency shift estimation of high dynamic signals based on ephemeris. After compensating the communication signal with error-containing frequency shift estimation, there is still a frequency offset residual at the receiving end. Therefore, it is necessary to establish a corresponding error analysis method to quantify the range of frequency offset residuals and guide the design of communication systems and waveform parameter configuration, that is, to reasonably compress the sweep width and reduce the number of sweeps according to the compensated carrier frequency offset residuals, and at the same time reduce the loop bandwidth of the digital phase-locked loop to improve the phase margin, so as to quickly synchronize and establish a communication link when performing actual communication tasks. Summary of the invention

[0005] The purpose of the present invention is to provide a Doppler frequency shift estimation and error estimation method for high dynamic signals based on ephemeris. The method is applicable to low-orbit satellite communication systems and supports Doppler frequency shift estimation and error analysis for low-orbit satellite-to-ground communication and inter-satellite communication.

[0006] The technical solution adopted by the present invention is:

[0007] A method for estimating Doppler frequency shift and error of a high dynamic signal based on ephemeris comprises the following steps:

[0008] (1) Select an ephemeris prediction model suitable for low-orbit satellites and obtain the ephemeris of the satellite; for satellite-to-ground communication, obtain the ephemeris of a single satellite; for inter-satellite communication, obtain the ephemeris of two satellites. The position of the ground station is known;

[0009] (2) Substituting the satellite's ephemeris parameter set into the ephemeris prediction model, the position and velocity of the satellite in the geocentric inertial coordinate system at any time during the communication window are obtained;

[0010] (3) Using the position and velocity of the two ends of the communication transmitter and receiver, combined with the carrier frequency, the relative velocity of the two ends is calculated according to the vector modulus formula, the cosine of the angle between the communication link and the relative motion direction of the two ends is calculated according to the vector angle formula, and the Doppler frequency shift estimate is calculated according to the Doppler effect of electromagnetic waves;

[0011] (4) Determine the carrier frequency error, relative velocity error, and angle error, and calculate the Doppler frequency shift estimation error within the communication window period according to the error propagation law.

[0012] Furthermore, the specific process of step (4) includes:

[0013] Determine the carrier frequency error Δf based on the performance indicators of the communication equipment c ;

[0014] Determine the relative velocity error Δv based on the accuracy of the ephemeris prediction model:

[0015]

[0016] In the formula, is the velocity accuracy of the transmitter, is the velocity accuracy of the receiving end; where, for the typical scenario of low-orbit satellite communicating with a fixed ground station, the relative rate error Δv is simplified as follows:

[0017] max|Δv|≤dr

[0018] In the formula, is the satellite velocity accuracy;

[0019] Determine the relative rate error angle error Δα based on the accuracy of the ephemeris prediction model:

[0020]

[0021] In the formula, dr T is the position accuracy of the transmitter, dr Ris the position accuracy of the receiving end; for the typical scenario of low-orbit satellite communicating with a fixed ground station, the relative rate error angle error Δα is simplified as follows:

[0022]

[0023] In the formula, dr is the satellite's position accuracy, h is the satellite's orbital height, and v is the satellite's moving speed;

[0024] According to the error propagation law, the Doppler frequency shift estimation error Δf within the communication window period is calculated D :

[0025]

[0026]

[0027] Where v is the relative speed between the two ends of the transmitter and receiver, cosα is the cosine of the angle between the communication link and the relative motion direction between the two ends of the transmitter and receiver, and f D is the Doppler shift estimate, f c is the carrier frequency.

[0028] The present invention has the following advantages:

[0029] The present invention supports Doppler frequency shift estimation and error analysis of low-orbit satellite-to-ground communication and inter-satellite communication; the error analysis is easy to implement; and the error analysis can be performed independently of the ephemeris model in typical scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of the Doppler frequency shift estimation and error estimation method of high dynamic signal based on ephemeris of the present invention. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below.

[0032] In the specific implementation, the typical scenario of low-orbit satellites and fixed ground stations is considered. The low-orbit satellite refers to the first phase of SpaceX's Starlink constellation, with an orbit altitude of about 550 km and a flight speed of 7.6 km / s. The communication signal carrier frequency f c =30GHz.

[0033] like Figure 1 As shown, a method for estimating Doppler frequency shift and error of a high dynamic signal based on ephemeris comprises the following steps:

[0034] (1) Select an ephemeris prediction model suitable for low-orbit satellites and obtain the satellite's ephemeris parameters.

[0035] Different ephemeris prediction models have different applicable scopes and input parameter groups. The ephemeris prediction model for the satellite in this embodiment selects the 4th version of the simple generalized perturbation (SGP4) model, whose input is two rows of orbital elements, which can be publicly consulted.

[0036] Select an ephemeris prediction model suitable for low-orbit satellites and choose an input parameter group that is as close to the communication window period as possible to ensure prediction accuracy.

[0037] For satellite-to-ground communications, it is necessary to obtain the ephemeris of a single satellite. The position of the ground station is known, and its velocity is 0 in the geocentric inertial coordinate system.

[0038] For inter-satellite communication, it is necessary to obtain the ephemeris of two satellites, and each satellite can select a different ephemeris prediction model.

[0039] (2) Predict the position and speed of the satellite during the communication window period based on the ephemeris prediction model.

[0040] The two-row orbital elements of the satellite are substituted into the ephemeris prediction model, and the major semi-axis, initial average velocity, atmospheric drag perturbation, non-spherical gravitational perturbation, long-period perturbation, short-period perturbation, and unit direction vector are calculated in turn, and finally the position and velocity of the satellite in the geocentric inertial coordinate system at any time during the communication window are obtained.

[0041] The position of the ground station is fixed and its velocity is 0 in the geocentric inertial coordinate system.

[0042] (3) Estimate Doppler shift.

[0043] Let f c is the carrier frequency, c is the speed of light, r T is the position coordinate vector of the transmitter, is the velocity coordinate vector of the transmitter, r R is the position coordinate vector of the receiving end, is the velocity coordinate vector of the receiving end, the subscript “T” represents transmission, and the subscript “R” represents reception.

[0044] The relative speed v between the transmitter and the receiver is calculated according to the vector modulus formula:

[0045]

[0046] According to the vector angle formula, calculate the cosine of the angle cosα between the communication link and the relative motion direction of the two ends of the transmitter and receiver:

[0047]

[0048] Calculate the Doppler frequency shift estimate f according to the Doppler effect of electromagnetic waves D :

[0049]

[0050] When implemented, the Doppler shift estimate f D ∈[-500KHz,+500KHz].

[0051] When transmitting or receiving, the signal is pre-applied with the Doppler frequency shift estimate f D The opposite frequency deviation is used to compensate for the Doppler effect.

[0052] (4) Analyze the Doppler frequency shift estimation error.

[0053] Let Δf c is the carrier frequency error, Δv is the relative rate error, and Δα is the angle error.

[0054] Analyze the Doppler frequency shift estimation error Δf during the communication window period according to the error propagation law D :

[0055]

[0056] Carrier frequency error Δf c It is related to the performance indicators of communication equipment and can be calibrated in advance.

[0057] The relative velocity error Δv and the angle error Δα are related to the accuracy of the ephemeris prediction model. Assume that the position accuracy of the transmitter is better than dr T , the velocity accuracy of the transmitter is better than The position accuracy of the receiving end is better than dr R , the speed accuracy of the receiving end is better than The position accuracy and speed accuracy can be obtained by taking empirical values ​​or combining theory and actual measurement.

[0058] The relative velocity error Δv satisfies:

[0059]

[0060] The angle error Δα satisfies:

[0061]

[0062] In the typical scenario of low-orbit satellites communicating with fixed ground stations, in order to simplify the process, the minimum value of the communication link min|r T -r R |You can take the satellite orbit altitude, relative velocity v and its minimum value The running speed corresponding to the current orbital altitude can be taken, while ignoring the influence of the position accuracy and velocity accuracy of the fixed ground station on the relative rate error and angle error. In this way, the error analysis can be carried out directly without relying on the results of the previous process steps.

[0063] In this embodiment, the carrier frequency error is calibrated. If it is not applicable, the empirical value max|Δf can be taken c |=1MHz.

[0064] The position accuracy and velocity accuracy of the SGP4 model are related to the orbital altitude. According to empirical values, when the orbital altitude is around 600 km, the position accuracy of the SGP4 model's short-term prediction is better than 1.2 km, and the velocity accuracy is better than 0.45 m / s.

[0065] The minimum value of the communication link is the orbital altitude, which is 550 km. The relative speed and its minimum value are the flight speed corresponding to the current orbital altitude, which is 7.6 km / s.

[0066] According to the above values, it can be concluded that max|Δv|≤0.45m / s, max|Δα|≤0.01rad.

[0067] When a low-orbit satellite communicates with the ground, the angle changes continuously from 0 to π or from π to 0. 101 points including the two endpoints are selected at equal intervals in the interval [0,π] as the angle value, and other parameter values ​​are combined to obtain the Doppler frequency shift estimation error Δf according to the error propagation law. D ≤5.3KHz. Compared with the Doppler frequency shift range, after compensating the Doppler frequency shift based on the ephemeris, the carrier frequency deviation residual is less than 2%.

[0068] In summary, the content of the present invention can complete the Doppler frequency shift estimation and error analysis of low-orbit satellite-to-ground communication and inter-satellite communication. The error analysis process is easy to implement, and in typical scenarios, the error analysis can be performed independently without relying on ephemeris model prediction.

Claims

1. A method for ephemeris-based Doppler frequency shift estimation and error estimation of high dynamic signals, characterized in that: The following steps are involved: (1) Select an ephemeris prediction model suitable for low-orbit satellites and obtain the ephemeris of the satellite; for satellite-to-ground communication, obtain the ephemeris of a single satellite; for inter-satellite communication, obtain the ephemeris of two satellites. The position of the ground station is known; (2) Substituting the satellite's ephemeris parameter set into the ephemeris prediction model, the position and velocity of the satellite in the geocentric inertial coordinate system at any time during the communication window are obtained; (3) Using the position and velocity of the two ends of the communication transmitter and receiver, combined with the carrier frequency, the relative velocity of the two ends is calculated according to the vector modulus formula, the cosine of the angle between the communication link and the relative motion direction of the two ends is calculated according to the vector angle formula, and the Doppler frequency shift estimate is calculated according to the Doppler effect of electromagnetic waves; (4) Determine the carrier frequency error, relative velocity error, and angle error, and calculate the Doppler frequency shift estimation error within the communication window period according to the error propagation law.

2. The method for ephemeris-based Doppler frequency shift estimation and error estimation of high dynamic signals according to claim 1, characterized in that: The specific process of step (4) includes: Determine the carrier frequency error Δf based on the performance indicators of the communication equipment c ; Determine the relative rate error Δv based on the accuracy of the ephemeris prediction model: In the formula, is the velocity accuracy of the transmitter, is the velocity accuracy of the receiving end; where, for the typical scenario of low-orbit satellite communicating with a fixed ground station, the relative rate error Δv is simplified as follows: max|Δv|≤dr In the formula, is the satellite velocity accuracy; Determine the relative rate error angle error Δα based on the accuracy of the ephemeris prediction model: In the formula, dr T is the position accuracy of the transmitter, dr R is the position accuracy of the receiving end; for the typical scenario of low-orbit satellite communicating with a fixed ground station, the relative rate error angle error Δα is simplified as follows: In the formula, dr is the satellite's position accuracy, h is the satellite's orbital height, and v is the satellite's moving speed; According to the error propagation law, the Doppler frequency shift estimation error Δf within the communication window period is calculated D : Where v is the relative speed between the two ends of the transmitter and receiver, cosα is the cosine of the angle between the communication link and the relative motion direction between the two ends of the transmitter and receiver, and f D is the Doppler shift estimate, f c is the carrier frequency.

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